We have identified a new common integration locus (designated int-3) for the mouse mammary tumor virus (MMTV) in MMTV (CZECHII) induced mammary tumors. Our current data show that 8 out of 44 CZECHII tumor DNAs contain a viral insertion at this site. MMTV integration at int-3 has been found to induce a 6kb species of RNA in addition to the previously described 2.4kb RNA species. Both RNAs correspond to DNA sequences adjacent to the int-3 locus. In 10 out of the 44 CZECHII tumor DNAs, the int-1 locus was found to be occupied by an MMTV genome. One of these tumor DNAs also had an insertion of the int-2 locus. Analysis of the int loci in tumor DNAs from CZECHII mice infected with another strain of MMTV as well as other strains of MMTV infected mice shows that the frequency of int activation is a function of the virus strain as well as the genetic background of the host. In other work we have found that MMTV infected CZECHII mice develop mammary gland hyperplastic aveolar nodules (HAN). Currently hyperplastic outgrowth (HOG) lines are being developed from MMTV infected CZECHII.